If you're working with LLVM and run into a bug, we definitely want to know
about it. This document describes what you can do to increase the odds of
getting it fixed quickly.

Basically you have to do two things at a minimum. First, decide whether the
bug crashes the compiler (or an LLVM pass), or if the
compiler is miscompiling the program (i.e., the
compiler successfully produces an executable, but it doesn't run right). Based
on
what type of bug it is, follow the instructions in the linked section to narrow
down the bug so that the person who fixes it will be able to find the problem
more easily.

Once you have a reduced test-case, go to the LLVM Bug Tracking
System and fill out the form with the necessary details (note that you don't
need to pick a category, just use the "new-bugs" category if you're not sure).
The bug description should contain the following
information:

All information necessary to reproduce the problem.

The reduced test-case that triggers the bug.

The location where you obtained LLVM (if not from our Subversion
repository).

More often than not, bugs in the compiler cause it to crash—often due
to an assertion failure of some sort. The most important
piece of the puzzle is to figure out if it is crashing in the GCC front-end
or if it is one of the LLVM libraries (e.g. the optimizer or code generator)
that has problems.

To figure out which component is crashing (the front-end,
optimizer or code generator), run the
llvm-gcc command line as you were when the crash occurred, but
with the following extra command line options:

-O0 -emit-llvm: If llvm-gcc still crashes when
passed these options (which disable the optimizer and code generator), then
the crash is in the front-end. Jump ahead to the section on front-end bugs.

-emit-llvm: If llvm-gcc crashes with this option
(which disables the code generator), you found an optimizer bug. Jump ahead
to compile-time optimization bugs.

If the problem is in the front-end, you should re-run the same
llvm-gcc command that resulted in the crash, but add the
-save-temps option. The compiler will crash again, but it will leave
behind a foo.i file (containing preprocessed C source code) and
possibly foo.s for each
compiled foo.c file. Send us the foo.i file,
along with the options you passed to llvm-gcc, and a brief description of the
error it caused.

The delta tool helps to reduce the
preprocessed file down to the smallest amount of code that still replicates the
problem. You're encouraged to use delta to reduce the code to make the
developers' lives easier. This website
has instructions on the best way to use delta.

If you find that a bug crashes in the optimizer, compile your test-case to a
.bc file by passing "-emit-llvm -O0 -c -o foo.bc".
Then run:

opt -std-compile-opts -debug-pass=Arguments foo.bc
-disable-output

This command should do two things: it should print out a list of passes, and
then it should crash in the same was as llvm-gcc. If it doesn't crash, please
follow the instructions for a front-end bug.

If this does crash, then you should be able to debug this with the following
bugpoint command:

bugpoint foo.bc <list of passes printed by
opt>

Please run this, then file a bug with the instructions and reduced .bc files
that bugpoint emits. If something goes wrong with bugpoint, please submit the
"foo.bc" file and the list of passes printed by opt.

If you find a bug that crashes llvm-gcc in the code generator, compile your
source file to a .bc file by passing "-emit-llvm -c -o foo.bc"
to llvm-gcc (in addition to the options you already pass). Once your have
foo.bc, one of the following commands should fail:

llc foo.bc

llc foo.bc -relocation-model=pic

llc foo.bc -relocation-model=static

If none of these crash, please follow the instructions for a
front-end bug. If one of these do crash, you should
be able to reduce this with one of the following bugpoint command lines (use
the one corresponding to the command above that failed):

bugpoint -run-llc foo.bc

bugpoint -run-llc foo.bc --tool-args
-relocation-model=pic

bugpoint -run-llc foo.bc --tool-args
-relocation-model=static

Please run this, then file a bug with the instructions and reduced .bc file
that bugpoint emits. If something goes wrong with bugpoint, please submit the
"foo.bc" file and the option that llc crashes with.

If llvm-gcc successfully produces an executable, but that executable doesn't
run right, this is either a bug in the code or a bug in the
compiler. The first thing to check is to make sure it is not using undefined
behavior (e.g. reading a variable before it is defined). In particular, check
to see if the program valgrinds clean,
passes purify, or some other memory checker tool. Many of the "LLVM bugs" that
we have chased down ended up being bugs in the program being compiled, not
LLVM.

Once you determine that the program itself is not buggy, you should choose
which code generator you wish to compile the program with (e.g. C backend, the
JIT, or LLC) and optionally a series of LLVM passes to run. For example:

bugpoint will try to narrow down your list of passes to the one pass
that causes an error, and simplify the bitcode file as much as it can to assist
you. It will print a message letting you know how to reproduce the resulting
error.

Similarly to debugging incorrect compilation by mis-behaving passes, you can
debug incorrect code generation by either LLC or the JIT, using
bugpoint. The process bugpoint follows in this case is to try
to narrow the code down to a function that is miscompiled by one or the other
method, but since for correctness, the entire program must be run,
bugpoint will compile the code it deems to not be affected with the C
Backend, and then link in the shared object it generates.

Special note: if you are debugging MultiSource or SPEC tests that
already exist in the llvm/test hierarchy, there is an easier way to
debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
will pass the program options specified in the Makefiles:

cd llvm/test/../../program
make bugpoint-jit

At the end of a successful bugpoint run, you will be presented
with two bitcode files: a safe file which can be compiled with the C
backend and the test file which either LLC or the JIT
mis-codegenerates, and thus causes the error.

To reproduce the error that bugpoint found, it is sufficient to do
the following:

Regenerate the shared object from the safe bitcode file:

llc -march=c safe.bc -o safe.cgcc -shared safe.c -o safe.so

If debugging LLC, compile test bitcode native and link with the shared
object: